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WDMWDMPiotr TurowiczPiotr Turowicz
Poznan Supercomputing and Networking CenterPoznan Supercomputing and Networking Center
piotrekpiotrek@@man.poznan.plman.poznan.pl
9-10 October 20069-10 October 2006
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Agenda
Dense Wavelength Division Multiplexing– The traditional and emerging challenges– How does DWDM work?– What are the enabling technologies?– The evolution of optical fibres
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Optical Networking Challenges
Faster
Further
More Wavelengths
Traditional Challenges
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Optical Networking Challenges
Faster
Further
More Wavelengths
Access(FTTN, FTTC, FTTH)
Switching
Muxing
Traditional Challenges Emerging Challenges
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What is a Wavelength Mux?
Time Division
Mux
Tributaries are sent in their own timeslots
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Time Division
Mux
Tributaries are sent in their own timeslots
Tributaries are buffered and sent when capacity is available
Statistical Mux
What is a Wavelength Mux?
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Time Division
Mux
Tributaries are sent in their own timeslots
Tributaries are buffered and sent when capacity is available
Statistical Mux
Tributaries are sent over the same fibre, but at different wavelengths
Wavelength Division Mux
Tributaries may arrive on different fibres, and at "grey" wavelengths
Electrical inputs
What is a Wavelength Mux?
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Early WDM Deployment
• Two transmission wavelengths, most common... 1310nm 1550nm
• Coupler used to combine streams into the fibre
• Splitter (another coupler) and filters used to separate and detect specific streams
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Dense WDM
• Many more than 2 channels!• Initial ITU Grid allows 32 channels with 100GHz Spacing• Proprietary systems with up to 160 channels are
currently available as slideware
How many channels?
Be very, very careful regarding manufacturer claims!
(c.f. Never ask a barber if he thinks you need a haircut)
Be very, very careful regarding manufacturer claims!
(c.f. Never ask a barber if he thinks you need a haircut)
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Why don't the streamson different wavelengths
get "mixed up"?
Why don't the streamson different wavelengths
get "mixed up"?
Question...
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Dense WDM:ITU Channel Spacing
1600 1700140013001200 1500
Atte
nuat
ion
(dB
/km
)
Wavelength (nm)
0.1
0.2
0.3
0.4
0.5
0.6
15
25
15
30
15
35
15
40
15
45
15
50
15
55
15
60
15
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ITU Channel Spacing 100GHz
(Currently)
ITU Channel Spacing 100GHz
(Currently)
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A Basic Answer
• Light is sent into the fibre on a very narrow range of wavelengths… A typical DFB laser peak width is ~10MHz (~1pm at 1500nm)
• Different channels are spaced so that they don't "overlap" In this context, "overlap" implies a power coupling (ie. interference)
between one channel and its neighbours Typical spacing "rule of thumb"…take the transmission rate in Gbps,
multiply by 2.5, and you have the minimum channel spacing in GHz (eg. 100GHz at 40Gbps)
Another "rule of thumb": each time you double the transmission rate or the number of channels, an additional 3dB of transmission budget is needed
• Need to know the range of available wavelengths in the fibre
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DWDM Channel Spacing
• Must have enough channel spacing to prevent interaction at a given transmission rate… 40Gbps 100GHz 10Gbps 25GHz 2.5Gbps 6GHz
• Must test lasers from large batch, ensure temperature stability, and include margins for component ageing
• Total range of wavelengths must be able to be consistently and reliably amplified by EDFA "Accepted" EDFA range is 1530 to 1565 nm (C-band)
• Must be aware of fibre limitations (see later)
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Why (and Where) DWDM?
• DWDM increases capacity on a given point to point linkBandwidth is multiplied by factor of 4, 8, 16 etc.
• Typical 1st generation DWDM is deployed in point to point topologies, over long-haul distances
• In Metro installations, there is an active debate between mesh and ring-based topologies
• Economics of Metro DWDM are not clear-cutOften is cheaper to deploy more fibre
These markets are… Changing rapidly Are sensitive to nature of installed fibre Are very sensitive to disruptive technologies
These markets are… Changing rapidly Are sensitive to nature of installed fibre Are very sensitive to disruptive technologies
…more later!
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DWDM Enabling Technologies
• The notion of "Service Transparency"• Laser sources• Receivers• Tuneable filters• Fibre gratings• Modulation and Modulators• Wavelength couplers and demuxers• Optical amplifiers• Points of flexibility
Optical Cross-Connect (OXC)
Optical Add-Drop Mux (OADM)
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Service Transparency
• Each Lambda can carry any serial digital service for which it has an appropriate physical interface SONET/SDH
Which can be carrying ATM, PoS and other services ESCON c.f. SCSI, which is a parallel communication channel
(parallel to serial converters are available for SCSI) Fast/Gigabit Ethernet
• Each channel can be transmitting at different rates
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Why Lasers?
• Lasers in general... High power output (compared to beam diameter) Narrow transmission spectrum High spatial quality beam (diffraction limited) Well-defined polarisation state
• Semiconductor lasers Small Size
To improve efficiency with fibre coupling
To allow high density port counts Industrial scale production
Needs lots of them!
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A Basic Semiconductor Laser
P
N
Reflective coating
Partially reflective coating
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How Do Lasers Work?
Electron
"Low" energy level
"High" energy level
Energy absorbed(pump)
Electron
"Low" energy level
"High" energy level
Energy emitted
Electrons exist in a stable "low" energy state until we pump in energy to promote
them to a higher state
High energy state is unstable and electron will soon decay back to the low energy state, giving out a characteristic level of
energy in the process
Characteristic energy
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A Laser Cavity
Reflective Surface
Reflective Surface
Atom in "high" energy state
Photon of characteristic energy
Atom in "low" energy state
Gain Medium
Atom will emit photon and return to "low" energy state.
The emitted photon has exactly the right energy to stimulate emission in the other high energy atoms
Photons that travel parallel to sides of resonant cavity are returned to stimulate further
emissions
Containment Layer
Electrodes
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Tuneable LasersWhat and Why?
• The ability to select the output wavelength of the laser… The primary sources are fixed wavelength
• What happens if one of these lasers fails? How many backup lasers would we need? What is the range of wavelengths over which we need to
operate?
• We could use one tuneable laser to back up all of the primary sources
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• There are three parameters that we trade-off in a tuneable laser… Tuning range (goal 35nm) Power output (goal 10mW) Settling latency (app. specific)
• Tunable lasers with a "slow" settling speed can be used in service restoration applications
• Tunable laser with a "fast" settling speed can also be used in next generation optical switching designs
Module 9831L Tuning Comb; Superimposed Spectra
-60
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
1525 1530 1535 1540 1545 1550 1555 1560 1565 1570 1575
Wavelength (nm)
Ou
tpu
t (d
B r
el.)
Tuneable LasersWhat and Why?
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Signal Modulation
• Notion of imposing a digital signal on a carrier wave Amplitude Modulation Frequency Modulation Phase Modulation
• In Optical Communications, typically Amplitude Modulation NRZ and RZ encoding
• Directly modulated lasers• Externally modulated lasers
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Modulation Schemes
• NRZ: non-return to zero Most common modulation
scheme for short-medium-long haul
• RZ: return to zero Ultra-long haul
0 00
1 1Signal
1
0 00
1 1Signal
1
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A Traditional Optical Repeater
•High speed electrical componentsHigh cost, lower reliability
•Single wavelength operation•Regenerator will make amplifier rate-specific
This system is not Service-Transparent!This system is not Service-Transparent!
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OEO Amps in a DWDM System
RX
RX
RX
Amp
Amp
Amp
TX
TX
TX
TX
TX
TX
Amp
Amp
Amp
RX
RX
RX
~40km
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Solution:Broadband, All-Optical Amplifier
•Single amplifier for multiple wavelengths•No electrical components
Cheaper, more reliable, not rate-dependent
Gain elementGain element
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The EDFAWhat is "Erbium Doped"?
CoreCore
CladdingCladding
• Fibre is "doped" with the element Erbium Controlled level of Erbium introduced into silica core
and cladding
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The EDFAHow Does It Work?
• Energy is "pumped" into the fibre using a pump laser operating at 980nm
• Erbium acts as lasing medium, energy transferred to signal
• Not specific to wavelength (operates in the EDFA Window)
• Not specific to transmission rate
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The EDFAHow Does It Work?
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The EDFA WindowRegion of "flat gain"
OH-
OH-
OH-
Wavelength (nm)
700 800 900 1000 1100 1200 1300 1400 1500 1600 1700
Att
enu
atio
n (
dB
/km
)
0
1
2
3
4
5
First windowSecond windowThird windowFourth windowFifth window
EDFA Window: 1530-1565nm
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CWDM
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CWDM
Coarse wavelength division multiplexing (CWDM)
is a method of combining multiple signals on laser beams at various wavelenghts for transmission along fiber optic cables, such that the number of chanels is fewer than in dense wavelength division multiplexing (DWDM) but more than in standard wavelength division multiplexing (WDM).
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CWDM
CWDM systems have channels at wavelengths spaced 20 nanometers apart, compared with 0.4 nm spacing for DWDM. This allows the use of low-cost, uncooled lasers for CWDM.
In a typical CWDM system, laser emissions occur on eight channels at eight defined wavelengths:
1610 nm, 1590 nm, 1570 nm, 1550 nm, 1530 nm, 1510 nm, 1490 nm, 1470 nm.
But up to 18 different channels are allowed, with wavelengths ranging down to 1270 nm
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CWDM
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CWDM
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CWDM
System CWDMCoarse Wavelength Division Multiplexing
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CWDM
System CWDMCoarse Wavelength Division Multiplexing
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The Evolution of Fibre
• Fibre properties Attenuation Dispersion Non-linearlity
• Fibre Evolution Dispersion-Unshifted Fibre (USF) Dispersion-Shifted Fibre (DSF) Non-Zero Dispersion-Shifted Fibre (NZDF) Emerging fibre types
• Soliton Dispersion Management
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Optical Fibre Properties
Faster
Further
More Wavelengths
Traditional Challenges Fibre Properties Attenuation Modal Dispersion Chromatic Dispersion Polarisation Mode
Dispersion Non-linearity
» Self-phase modulation» Cross-phase
modulation» 4-wave mixing
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Fibre Optic PropertiesSignal Attenuation
OH-
OH-
OH-
~190THz
~50THz
Wavelength (nm)
700 800 900 1000 1100 1200 1300 1400 1500 1600 1700
Att
enu
atio
n (
dB
/km
)
0
1
2
3
4
5
First windowSecond windowThird windowFourth windowFifth window
1
2 3 45
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Fibre Optic PropertiesModal Dispersion
• In multimode cable, different modes travel at different speeds down the fibre
Result: signal is "smeared" Solution: single mode fibre
Signal in Signal out
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Fibre Optic PropertiesChromatic Dispersion
Different wavelengths travel at different speeds down the cable
Result: signal is "smeared" Solution: narrow spectrum lasers Solution: avoid modulation chirp Solution: dispersion management
Signal in Signal out
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Fibre Optic PropertiesPolarisation Mode Dispersion
Different polarisation components travel at different speeds down the cable
Result: signal is "smeared" Solution: design and installation experience, good test equipment
Slow
Fast PMD delay timePulses start journey in phase
After travelling down fibre, pulses are now
out of phase
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Fibre Optic PropertiesNon-Linear Effects
• Self Phase Modulation• Cross Phase Modulation• 4-Wave Mixing
Effects are "triggered" when power level of signal exceeds a
certain threshold
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Self Phase Modulation (SPM)
• Non-linear effect• Occurs in single and multi
wavelength systems In DWDM system, SPM will
occur within a single wavelength
• Two main effects… Spectral broadening Pulse compression
• Solution is positive dispersion in signal path In
ten
sity
Time
Spectral broadening
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Cross-Phase Modulation (XPM)
• Pulses in adjacent WDM channels exchange power ie. only happens in multi-
channel systems
• Primary effect is spectral broadening
• Combined with high dispersion, will produce temporal broadening
• Low levels of positive dispersion will help prevent inter-channel coupling
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Four Wave Mixing
Case 1: Intensity modulation between two primary channels at beat frequency
Result is two "phantom" wavelengths
Case 2: Interaction of three primary frequenciesResult is a "phantom" fourth wavelength
fF = fp + fq - fr
f1 f22f1-f2 2f2-f1fF
fp fq fr
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Fibre Evolution1st Generation: USF
1300 1400 1500 1600
-20
-10
0
10
20
Wavelength (nm)
Dis
per
sio
n (
ps/
nm
-km
)1310nm
Att
enu
atio
n (
dB
/km
)
0.2
0.3
0.4
0.5
Dispersion
USF
1550nm
Attenuation
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Fibre Evolution2nd Generation: DSF
1300 1400 1500 1600
-20
-10
0
10
20
Wavelength (nm)
Dis
per
sio
n (
ps/
nm
-km
)
Att
enu
atio
n (
dB
/km
)
0.2
0.3
0.4
0.5
Dispersion
USF
DSF
Attenuation
1310nm 1550nm
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Fibre Evolution3nd Generation: NZDSF
1300 1400 1500 1600
-20
-10
0
10
20
Wavelength (nm)
Dis
per
sio
n (
ps/
nm
-km
)
Att
enu
atio
n (
dB
/km
)
0.2
0.3
0.4
0.5
Dispersion
USF
DSF
Attenuation
1310nm 1550nm
NZDF
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Next Generation Fibres...
• Remove OH- interaction to open 5th window Example: Lucent "All Wave" Fibre
• Minimise intrinsic PMD during manufacture PMD is the "2.5Gbps speed bump" Example: Corning LEAF PMD is very dependent on installation stresses
• Reduce loss at higher wavelengths (>1600nm) Selctive doping using chalcogenides
(Group VI elements) Fibre bend radius becomes significant
53http://www.porta-http://www.porta-optica.orgoptica.org
Reichle & De-Massari
References
